What is the role of osmolar gap testing in the workup of methanol toxicity?

Updated: Nov 05, 2018
  • Author: Kalyani Korabathina, MD; Chief Editor: Tarakad S Ramachandran, MBBS, MBA, MPH, FAAN, FACP, FAHA, FRCP, FRCPC, FRS, LRCP, MRCP, MRCS  more...
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Methanol ingestion results in an elevated osmolar gap, so in cases of stupor of unknown cause, testing for an osmolar gap should be routine. However, the osmolar gap is a nonspecific finding because it may represent the presence of a low ̶ molecular weight solute, such as ethanol, other alcohols, mannitol, glycine, lipids, or proteins.

The osmolar gap can be calculated using a set formula. To find the osmolar gap, take the measured plasma osmolality and subtract the calculated osmolality. Calculated osmolality requires a serum glucose measurement and is derived as follows:

Calculated osmolality (mOsm/kg) = 2(Na+) + (glucose/18) + (blood urea nitrogen [BUN]/2.8)

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